The present invention relates to sample preparation and analysis. More specifically, the invention relates to integrated microdevices for preparing and introducing a small volume of a fluid sample into an ionization chamber of an analytical device, such as a mass spectrometer, an absorption spectrometer or an emission spectrometer. The invention also relates to methods for sample introduction using the novel integrated microdevices.
Atomic or elemental analysis techniques allow for precise measurements of minute quantities of sample materials. Common analytical techniques include mass spectrometry, inductively coupled plasma spectrometry, inductively coupled plasma atomic emission spectrometry, and so forth. Elemental analysis by mass spectrometry is a generally well established technique. Inductively coupled plasma mass spectrometry (ICP-MS), in particular, is a powerful elemental analysis tool used in a variety of applications, such as environmental, geological, semiconductor and biological sample analyses. Various aspects of plasma mass spectrometry technology are described in patents such as in U.S. Pat. No. 5,334,834 to Ito et al., U.S. Pat. No. 5,519,215 to Anderson et al., and U.S. Pat. No. 5,572,024 to Gray et al. For example, U.S. Pat. No. 5,334,834 to Ito et al. describes a device for controlling the plasma potential in an ICP-MS. In ICP-based methods, the test sample is typically converted into an aerosol and transported into a plasma where desolvation, vaporization, atomization, excitation and ionization processes occur.
For fluid samples, sample introduction is a critical factor that determines the performance of analytical instrumentation such as a mass spectrometer. Analyzing the elemental constituents of a fluid sample generally requires the sample to be dispersed into a spray of small droplets. For instance, in mass spectrometry, atomic emission spectrometry or atomic absorption spectrometry, the sample is ionized. In ordinary ICP-MS, a combination of a nebulizer and a spray chamber is used in sample introduction because of the simplicity and relative low cost of the combination. The nebulizer produces the spray of droplets and the droplets are then forced through a spray chamber and sorted. However, use of this combination only introduces a small fraction of the aerosol into the plasma of the ICP-mass spectrometer because the larger droplets may condense on the walls of the spray chamber. As a result, this combination suffers from low analyte transport efficiency and high sample consumption. In addition, the use of the combination produces a memory effect, i.e., the sample signal will persist for a long period after the sample introduction is over (more particularly, xe2x80x9cmemory effectxe2x80x9d may be defined to encompass the persistence of a signal as a result of release of adsorbed or residual fluid sample in either any portion a nebulizer or spray chamber). This analyte carry-over memory phenomenon in ICP-MS has been described, e.g., in U.S. Pat. No. 6,002,097 Morioka et al. The memory effect is especially problematic when a mass spectrometer is employed to analyze different fluid samples in sequence. Cross contamination compromises analytical results. Consequently, efforts in improving sample introduction for ICP-MS have focused on increasing spray efficiency and reducing memory effect. To obtain accurate and reliable results from an instrument that has the aforementioned memory effect, sufficient time must be provided to allow for a wash-out before a subsequent sample can be introduced. For these reasons, the throughput of instruments such as ICP-mass spectrometers using a combination of a nebulizer and a spray chamber has previously been low.
Many nebulization methods and devices are currently known in the art and include pneumatic, ultrasonic, direct injection, high-efficiency and electrospray nebulization. Two different geometries are the most common in pneumatic nebulization: the concentric type and the cross flow (including V-groove and Babington) type. Some nebulizers employ multiple nebulization methods. For example, an electrospray nebulizer may include an electrospray needle having a concentric gas flow. A concentric nebulizer with a small orifice (i.e., a microconcentric nebulizer) has been successfully used to increase spray efficiency, but tends to clog when spraying samples with a high concentration of dissolved solids. The direct injection nebulizer (DIN) is useful for reducing memory effect. It is also useful when the amount of the sample is limited or when maintaining the spatial or temporal resolution of chemical species is important, such as when coupling liquid chromatography (LC) or capillary electrophoresis (CE) to ICP-MS. However, none of these approaches correct for all known problems associated with nebulization.
It is clear, then, that the performance of a sample introduction system is evaluated with regard to parameters such as transport efficiency, precision, reproducibility, reliability, detection limits, sample size demand, liquid flow demand, spectral and nonspectral interference and wash-out time. The following patents and publications describe various aspects of sample introduction systems.
Published reports of nebulization methods and devices include Tangen et al., xe2x80x9cMicroconcentric nebulizer for the coupling of micro liquid chromatography and capillary zone electrophoresis with inductively coupled plasma mass spectrometry,xe2x80x9d JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, 1997, 12(N6):667-670; Taylor et al., xe2x80x9cDesign and characterisation of a microconcentric nebuliser interface for capillary electrophoresis-inductively coupled plasma mass spectrometry,xe2x80x9d JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, 1998, 13(N10):1095-1100; and Mclean, J. A. et al., xe2x80x9cA direct injection high-efficiency nebulizer for inductively coupled plasma mass spectrometry,xe2x80x9d ANALYTICAL CHEMISTRY, 1998, 70(N5):1012-1020; Kirlew et al., xe2x80x9cInvestigation of a modified oscillating capillary nebulizer design as an interface for CE-ICP-MS,xe2x80x9d APPLIED SPECTROSCOPY, 1998, 52(N5):770-772; and Haraguchi et al., xe2x80x9cSpeciation of yttrium and lanthanides in natural water by inductively coupled plasma mass spectrometry after preconcentration by ultrafiltration and with a chelating resin,xe2x80x9d ANALYST, 1998, 123(N5):773-778.
Ultrasonic energy has also been used to nebulize samples, and such use has been described in such publications as Kirlew et al., xe2x80x9cAn evaluation of ultrasonic nebulizers as interfaces for capillary electrophoresis of inorganic anions and cations with inductively coupled plasma mass spectrometric detection,xe2x80x9d SPECTROCHIMICA ACTA PART B-ATOMIC SPECTROSCOPY, 1998, 53(N2):221-237.
U.S. Pat. No. 5,868,322 to Loucks et al. describes methods and systems for nebulization of samples and for introduction of the samples into gas-phase or particle detectors. The patent describes a device having an outer tube and at least one inner tube, with fluid sample flowing out of the inner tube(s) during use. Either gas or liquid may flow in the outer tube. Liquid flowing in the outer tube may serve as xe2x80x9cmake-up fluidxe2x80x9d and may also serve to stabilize flow in a buffer region.
U. S. Pat. No. 5,259,254 to Zhu et al. describes a method and system for nebulizing liquid samples and introducing the resulting sample droplets into a sample analysis system. Nebulization is performed with an ultrasonic nebulizer comprising a piezoelectric crystal or an equivalent ultrasound source covered with a barrier, such as a polyimide film, which serves as an interface between the ultrasound source and a heat sink. The system further comprises a solvent removal system. Any gas phase or particle sample analysis system may be used, including ICP-MS.
In addition, samples separated by high performance liquid chromatography have been nebulized and introduced into atomic emission spectrometers, as is disclosed in Elgersma et al., xe2x80x9cElectrospray as interface in the coupling of micro high-performance liquid chromatography to inductively coupled plasma atomic emission spectrometry,xe2x80x9d JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, 1997, 12(N9):1065-1068 and Raynor et al., xe2x80x9cElectrospray nebulisation interface for micro-high performance liquid chromatography inductively coupled plasma mass spectrometry,xe2x80x9d JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, 1997, 12(N9):1057-1064.
The sample separation resulting from ion chromatography has been analyzed by Inductively Coupled Plasma Atomic Emission spectroscopy (ICP/AE). For example, see Harwood et al., xe2x80x9cAnalysis of organic and inorganic selenium anions by ion chromatography inductively coupled plasma atomic emission spectroscopy,xe2x80x9d JOURNAL OF CHROMATOGRAPHY A, 1997, 788(N1-2):105-111. In addition, the output of capillary electrophoresis has been analyzed by Hagege et al., xe2x80x9cOptimization of capillary zone electrophoresis parameters for selenium speciation,xe2x80x9d MIKROCHIMICA ACTA, 1997, 127(N1-2):113-118.
Coupling the output of a sample separation device, such as CE or HPLC, with the input of an elemental analysis device allows one to analyze the separated components of a sample with great precision. It is recognized in the art that such coupling offers many advantages; the topic is discussed, for example, in Mass Spectrometry Principles and Applications by de Hoffman et al., Chapter 3. In addition, U.S. Pat. No. 5,597,467 to Zhu et al., describes a system for interfacing capillary electrophoresis (CE) with ICP-MS that includes a sample introduction tube as an integral part of the sample introduction device. Sample introduction into the ICP-MS is via a direct injection nebulizer. Injected sample is mixed with conductive xe2x80x9cmake-upxe2x80x9d liquid before nebulization in order that the separation of the sample components effected by CE will not be altered by flow to and through the nebulizer. In addition, the make-up liquid serves as part of the circuit pathway for creating the voltage gradient necessary for CE.
In many cases, analytical devices using nebulizers that process a large volume of sample exhibit a high degree of contamination, fouling or clogging. Residue may build up over time; such build-up is exacerbated by the larger the volume of sample placed into the analysis device. In contrast, when only small amounts of sample are available, clogging is not as problematic. Thus, devices requiring smaller sample amounts are desired.
Currently, microfabricated devices have been used as chemical analysis tools as well as clinical diagnostic tools. Their small size allows for the analysis of minute quantities of sample, which is an advantage where the sample is expensive or difficult to obtain. See, for example, U.S. Pat. No. 5,500,071 to Kaltenbach et al., U.S. Pat. No. 5,571,410 to Swedberg et al., and U.S. Pat. No. 5,645,702 to Witt et al. Sample preparation, separation and detection compartments have been proposed to be integrated on such devices. However, the production of such devices present various challenges. For example, the flow characteristics of fluids in the small flow channels of a microfabricated device may differ from the flow characteristics of fluids in larger devices, as surface effects come to predominate and regions of bulk flow become proportionately smaller.
Accordingly, a device is desired that requires only small volumes of sample, and does not suffer from memory effect or cross contamination and does not require long washing times. It would be advantageous to apply the sensitive analytical techniques of elemental analysis to the separated samples provided by microfabricated devices. Accordingly, new and improved sample introduction technologies are in demand for elemental analysis methods such as ICP-MS, especially when the sample amount is limited, the sample concentration is extremely low, the sample has both high concentration and low concentration components (high dynamic range), the sample is in a complex matrix, speciation information is needed for the sample and/or high sample throughput is required. The use of disposable integrated microfabricated devices as sample introduction tools for ICP-MS offer many advantages in solving such problems.
Accordingly, it is an object of the present invention to overcome the above-mentioned disadvantages of the prior art by providing a microdevice for introducing a fluid sample into an ionization chamber.
It is another object of the invention to provide such a microdevice wherein the fluid sample is nebulized before entering the ionization chamber.
It is still another object of the invention to provide such a microdevice that is disposable and/or detachable from the ionization chamber.
It is a further object of the invention to provide such a microdevice that further comprises an integrated nebulizer and/or other integrated features for performing chemical or biochemical reactions to prepare the fluid sample for introduction into the ionization chamber.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by routine experimentation during the practice of the invention.
In a general aspect, then, the present invention relates to a microdevice for introducing a fluid sample into an ionization chamber. The microdevice includes a substrate having a first and second opposing surfaces, wherein a microchannel is formed in the first surface of the substrate. A cover plate is arranged over the first surface, and the cover plate in combination with the microchannel defines a conduit for conveying the sample. A sample inlet port is provided in fluid communication with the microchannel. The inlet port allows the fluid sample from an external source to be conveyed in a defined sample flow path that travels, in order, through the inlet port, the conduit and a sample outlet port and into the ionization chamber. Adjacent to the sample outlet port is a nebulizing region in which a nebulizing means nebulizes the fluid sample.
In another aspect, the invention relates to the above microdevice, wherein the nebulizing means comprises a nebulizing gas source in gaseous communication with the nebulizing region, and further wherein the nebulizing region is adapted to allow a nebulizing gas from the gas source to nebulize the fluid sample. The nebulizing means may represent an integrated portion of the microdevice.
In still another aspect, the invention relates to the above microdevice further comprising a sample preparation portion for preparing the fluid sample. The sample preparation portion may be in downstream fluid communication with the inlet port such that sample flow path travels, in order, through the inlet port, the sample preparation portion and the outlet port. The sample preparation portion may be adapted to serve as a reaction zone for carrying out a chemical reaction with the fluid sample. In the alternative or in addition, the sample preparation portion may be adapted to separate the fluid sample into a plurality of constituents at least one of which is conveyed to the sample outlet port. Separation may be carried out using a separation means selected from the group consisting of capillary electrophoresis means, chromatographic separation means, electrochromatographic separation means, electrophoretic separation means, hydrophobic interaction separation means, ion exchange separation means, iontophoresis means, reverse phase separation means, and isotachophoresis separation means. As a further alternative, the sample preparation portion may comprise a plurality of sample preparation chambers, each chamber adapted to alter a property of the fluid sample, e.g., temperature, chemical composition, purity and concentration.
In yet another aspect, the invention relates to the above microdevice, wherein the sample preparation portion comprises a plurality of sample preparation chambers, each chamber adapted to alter a property of the fluid sample. The plurality of sample preparation chambers may comprise a reaction chamber in upstream fluid communication with a separation chamber.
In a further aspect, the invention relates to the above microdevice further comprising an attachment portion adapted for releasable attachment with the ionization chamber. Such a microdevice may be disposable or adapted for multiple use.
In a still further aspect, the invention relates to the above microdevice, wherein the substrate is composed of a polymeric material. The polymeric material may be selected from the group consisting of polyimides, polycarbonates, polyesters, polyamides, polyethers, polyurethanes, polyfluorocarbons, polystyrenes, poly(acrylonitrile-butadiene-styrene)(ABS), acrylate and acrylic acid polymers such as polymethyl methacrylate, and other substituted and unsubstituted polyolefins, and copolymers thereof.
In another aspect, the invention relates to the above microdevice, wherein the sample preparation portion is sized to contain approximately 1 xcexcl to 500 xcexcl of fluid, or preferably approximately 10 xcexcl to 200 xcexcl of fluid.
In still another aspect, the invention relates to the above microdevice, wherein the microchannel is approximately 1 xcexcm to 200 xcexcm in diameter, preferably approximately 10 xcexcm to 75 xcexcm in diameter.
In a further aspect, the invention relates to the above microdevice, wherein any one of the microchannel, sample inlet port or sample outlet port is formed through laser ablation, embossing, injection molding, or a LIGA process.
In a still further aspect, the invention relates to the above microdevice, wherein the ionization chamber represents a component of an inductively coupled plasma mass spectrometer.
In another general aspect, the invention relates to a method for introducing a fluid sample into an ionization chamber. The method involves: (a) providing a microdevice comprising a substrate having a first and second opposing surfaces, the substrate having a microchannel formed in the first surface, a cover plate arranged over the first surface, the cover plate in combination with the microchannel defining a conduit for conveying the sample and a sample inlet port in fluid communication with the microchannel, wherein the sample inlet port allows the fluid sample from an external source to be conveyed in a defined sample flow path that travels, in order, through the sample inlet port, the conduit and a sample outlet port and into the ionization chamber of an inductively coupled plasma mass spectrometer; (b) injecting the fluid sample into the sample inlet port; (c) conveying the fluid in the defined sample flow path to the ionization chamber. The method may be useful in carrying out analysis of a fluid sample in an inductively coupled plasma mass spectrometer, wherein a mass spectrum is produced according to the mass of the sample ions.